1. Field of the Invention
The present invention relates to conformationally constrained parathyroid hormone (PTH) analogs, and methods of preparing and using the PTH analogs.
2. Background Art
Parathyroid Hormone
Parathyroid hormone (PTH), an 84 amino acid peptide, is the principal regulator of ionized blood calcium in the human body (Kronenberg, H. M., et al., In Handbook of Experimental Pharmacology, Mundy, G. R., and Martin, T. J., (eds), pp. 185-201, Springer-Verlag, Heidelberg (1993)). Regulation of calcium concentration is necessary for the normal function of the gastrointestinal, skeletal, neurologic, neuromuscular, and cardiovascular systems. PTH synthesis and release are controlled principally by the serum calcium level; a low level stimulates and a high level suppresses both hormone synthesis and release. PTH, in turn, maintains the serum calcium level by directly or indirectly promoting calcium entry into the blood at three sites of calcium exchange: gut, bone, and kidney. PTH contributes to net gastrointestinal absorption of calcium by favoring the renal synthesis of the active form of vitamin D. PTH promotes calcium resorption from bone indirectly by stimulating differentiation of the bone-resorbing cells, osteoclasts. It also mediates at least three main effects on the kidney: stimulation of tubular calcium reabsorption, enhancement of phosphate clearance, and promotion of an increase in the enzyme that completes synthesis of the active form of vitamin D. PTH is thought to exert these effects primarily through receptor-mediated activation of adenylate cyclase and/or phospholipase C.
Disruption of calcium homeostasis may produce many clinical disorders (e.g., severe bone disease, anemia, renal impairment, ulcers, myopathy, and neuropathy) and usually results from conditions that produce an alteration in the level of parathyroid hormone. Hypercalcemia is a condition that is characterized by an elevation in the serum calcium level. It is often associated with primary hyperparathyroidism in which an excess of PTH production occurs as a result of a parathyroid gland lesion (e.g., adenoma, hyperplasia, or carcinoma). Another type of hypercalcemia, humoral hypercalcemia of malignancy (HHM) is the most common paraneoplastic syndrome. It appears to result in most instances from the production by tumors (e.g., squamous, renal, ovarian, or bladder carcinomas) of a class of protein hormone which shares amino acid homology with PTH. These PTH-related proteins (PTHrP) appear to mimic certain of the renal and skeletal actions of PTH and are believed to interact with the PTH receptor in these tissues.
Osteoporosis
Osteoporosis is a potentially crippling skeletal disease observed in a substantial portion of the senior adult population, in pregnant women and even in juveniles. The term osteoporosis refers to a heterogeneous group of disorders. Clinically, osteoporosis is separated into type I and type II. Type I osteoporosis occurs predominantly in middle aged women and is associated with estrogen loss at menopause, while osteoporosis type II is associated with advancing age. Patients with osteoporosis would benefit from new therapies designed to promote fracture repair, or from therapies designed to prevent or lessen the fractures associated with the disease.
The disease is marked by diminished bone mass, decreased bone mineral density (BMD), decreased bone strength and an increased risk of bone fracture. At present, there is no effective cure for osteoporosis, though estrogen, calcitonin and the bisphosphonates, etidronate and alendronate are used to treat the disease with varying levels of success. These agents act to decrease bone resorption. Since parathyroid hormone regulates blood calcium and the phosphate levels, and has potent anabolic (bone-forming) effects on the skeleton, in animals (Shen, V., et al., Calcif. Tissue Int. 50:214-220 (1992); Whitefild, J. F., et al., Calcif. Tissue Int. 56:227-231 (1995) and Whitfield, J. F., et al., Calcif Tissue Int. 60:26-29 (1997)) and humans (Slovik, D. M., et al., J. Bone Miner. Res. 1:377-381 (1986); Dempster, D. W., et al., Endocr. Rev. 14:690-709 (1993) and Dempster, D. W., et al., Endocr. Rev. 15:261 (1994)) when administered intermittently, PTH, or PTH derivatives, are prime candidates for new and effective therapies for osteoporosis.
PTH Derivatives
PTH derivatives include polypeptides that have amino acid substitutions or are truncated relative to the full length molecule. Both a 14 and a 34 amino acid amino-terminal truncated form of PTH, as well as a C-terminal truncated form have been studied. Additionally, amino acid substitutions within the truncated polypeptides have also been investigated.
Synthetic PTH(1-34) exhibits full bioactivity in most cell-based assay systems, has potent anabolic effects on bone mass in animals and has been shown to reduce the risk of bone fracture in postmenopausal osteoporotic women (Neer, R. M., et al., N.E.J.M. 344:1434-1441 (2001); Dempster, D. W., et al., Endocr Rev 14:690-709 (1993)). PTH acts on the PTH/PTHrP receptor P1R), a class II G protein-coupled heptahelical receptor that couples to the adenylyl cyclase/cAMP and phospolipase C/inositol phosphate (IP) signaling pathway (Rippner, H., et al., Science 254:1024-1026 (1991)). Deletion analysis studies have shown that the amino-terminal residues of PTH play a crucial role in stimulating the P1R to activate the cAMP and IP signaling pathways (Tregear, G. W., et al., Endocrinology 93:1349-1353 (1973); Takasu, H., et al., Biochemistry 38:13453-13460 (1999)). Crosslinking and receptor mutagenesis studies have indicated that residues in the amino-terminal portion of PTH interact with the extracellular loops and extracellular ends of the seven transmembrane helices, which reside within the juxtamembrane region of the receptor (Bergwitz, C., et al., J. Biol. Chem. 271:26469-26472 (1996); Hoare, S. R. J., et al., J. Biol. Chem 276:7741-7753 (2001); Behar, V., et al., J. Biol. Chem. 275:9-17 (1999); Shimizu, M., et al., J. Biol. Chem. 275:19456-19460 (2000); Luck, M. D., et al., Molecular Endocrinology 13:670-680 (1999)).
α-Helix Stabilizers
The first 34 amino acids of PTH and PTHrP contain sufficient information for high affinity P1R binding and potent induction of P1R-mediated signaling responses (Neer, R M, et al., N.E.J.M. 344: 1434-1441 (2001)). Short N-terminal fragments of PTH, such as PTH(1-14) and PTH(1-11) exhibit extremely weak binding affinities (Kd>>100 μM) but are nonetheless capable of eliciting cAMP-signaling responses, albeit with potencies (EC50s≧100 μM) that are substantially weaker than that of PTH(1-34)(EC50˜2 nM) (Luck, M D et al., Molecular Endocrinology 13:670-680 (1999)). It has been reported that a series of modified PTH(1-14) and PTH(1-11) analogs exhibit signaling potencies that are nearly, or even fully, equal to that of PTH(1-34) (Shimizu, M et al., Endocrinology 142: 3068-3074 (2001); Shimizu, M. et al., J. Biol. Chem. 276:490003-49012 (2001); Shimizu, M. et al., J. Biol. Chem. 275: 21836-21843 (2000)).
Recently, it was also reported that PTH(1-14) analogs containing the α,α-disubstitued amino acid, α-aminoisobutyric acid (Aib) at positions 1 and/or 3, have 10- to 100-fold higher affinities and cAMP signaling potencies than do their counterpart peptides containing alanine at these positions (Shimizu, N. et al. J. Biol. Chem. 276: 49003-49012 (2001)).